Thursday, November 17, 2011

VENOMOUS SNAKES
REPTILES: Sri Lanka, though an island of relatively small size, is one of the richest countries in herpetological wealth in South Asian region. This rich diversity of herpetofauna consists of approximately 100 amphibian and 190 reptile species. Of these two groups, nearly 60% of species are endemic.

There are 294 species of snakes in the world of which, as mentioned above, 96 are found in Sri Lanka. Of these, 50 species are endemic to the island. There are 13 species of sea snakes and 10 species of blind snakes in Sri Lanka.

Indian Krait

Some snakes are found in all parts of the country from the seas surrounding the island to the highest point - Horton Plains. Some snakes are confined to specific geographic and climatic locations. Snakes vary in colour patterns from dull to highly ornamental.

The snake's skin is covered with scales, which are markedly wider underneath where they form. It is an essential aid for movement for all species except a few. The scales act like tire treads, gripping the ground and giving the snake the traction necessary to push itself forward.

Thus a snake is helpless on glass where these scales cannot get a grip on the smooth surface and propel its body. A snake has an undulating movement, a concertina type movement or a creeping movement or a combination of two of these movements.

Also, the scales underneath are hard and protect the snake as it moves along rough surfaces. The scales covering the body prevent excessive dehydration by retaining body moisture and contain pigments that form the color patterns of snakes.

Its eyesight is comparatively poor, though it has a hazy idea of movement in front. Its hearing is limited to picking up vibrations from the ground. It cannot pick up sound waves as other animals do.

Snakes have an acute sense of touch. Besides the sense of smell through the nasal passages, the flickering tongue picks up and taste airborne particles, which are then passed on to the Jacobson's Organ at the roof of the mouth for investigation.

Some snakes, for example, the Hump-nosed Viper and the Green Pit Viper, have a cavity between eye and nostril, which is very sensitive to infra-red rays and useful in locating warm-blooded prey in the dark.

All snakes are carnivorous. The diet of a snake varies between species and also depends on their size and behavior.

For example, small snakes typically prey on small, slow-moving animals like slugs and earthworms. Larger snakes often prey on larger, more active prey. Since snakes have no claws or chewing teeth, they have to swallow their prey as a whole.

They are able to swallow prey larger than their head by extending their jaws while swallowing and subsequently getting the jaws together when the swallowing process is finished. Snakes lack appendages to grip and hold prey.

As a result, many species have developed intriguing methods for capturing, immobilizing and killing their prey. Some snakes are able to kill their prey by constriction, thereby suffocating their victim, while others make use of venom that immobilizes their prey.

Some snakes are oviparous and others ovoviviparous. Ovoviviparous snakes lay eggs whilst viviparous snakes retain the eggs in the oviducts until development of the young is complete. In both cases the young are immediately self-sufficient.

Snakes are often camouflaged for better concealment in hunting as well as for their own protection. Snakes have evolved a variety of defense strategies since mammals, birds, and even other snakes prey them upon.

Camouflage coloration may conceal a snake's shape and confuse predators. When threatened, some snakes become immobile or "freeze" as they sense danger.

Only a few species of snakes are considered extremely venomous and have a venom that can kill humans The six most venomous terrestrial snakes in Sri Lanka are the Cobra (Naja naja), Russell's Viper (Daboia Russelli), Common Krait (Bungarus caeruleus), Sri Lankan Krait (Bungarus ceylonicus) and the Saw-scaled Viper (Echis carinatus).
The venom of these snakes can kill a human being. The Hump-nosed Viper (Hypnale hypnale) is called Polon thelissa in Sinhala and Kuzhi Viriya in Tamil. The Green Pit Viper (Trimeresurus trigonocephalus) is called Pala Polonga in Sinhala and Kopi Viriyan in Tamil. These snakes are thought by some to be very venomous but they are not.

Only three species account for 98% of the human deaths in the country. They are the Cobra, Russell's viper and Common Krait. Most snakes are quite harmless and non aggressive. When a venomous snake bites its prey, due to the action of the venom, the prey is paralyzed. This is the main function of the venom. The venom also helps in the digestion of its food.

Different species of snakes have venom of varying strength. There are two types of venom that snakes carry. One is called neurotoxic venom. These poisons attack the nervous system and also paralyze the muscles of the heart and or the respiratory system.

The venom of the cobra and the Kraits are neurotoxic. The other type of venom is haematoxic and gets into the circulation system of the blood and clots the blood. It also destroys the capillary walls. The venom of the Russell's viper is haemotoxic.

Snakes bite humans purely as a defensive measure. They have no intention whatsoever of consuming humans. The reaction of the human body to the poison depends on the strength or toxicity of the venom of that particular species of snake.

The strength of the venom depends on the chemical composition and the quantity contained in its venom gland at that time. The venom that the Green Pit Viper and the Cat snakes carry does not produce a fatal reaction in humans.

Though the Python (Python molurus) can inflict a bite causing severe lacerations but not death, it could kill a human by asphyxiation, tightening its coils around the throat of the victim.

Sri Lanka Krait

Within Sri Lankan borders, Sea Snakes or marine snakes (Hydrophiidae) possess the most lethal venom of all species. Yet due to their inoffensive nature, little or no casualties are on record. There are approximately 13 species of marine snakes, which inhabit the coastal waters and estuaries around the island.

However, Anslem de Silva, the renowned Sri Lankan herpetologist is of the opinion, according to his ongoing research, that there could be at least 20 species of sea snakes.

When compared to some other countries in South East Asia, the deaths due to sea snake bite are very rare in this country. There are records of one tourist being bitten by a sea snake. It is usually the fishermen who get bitten by sea snakes.

The most venomous terrestrial snakes of Sri Lanka are described below.

The Russell's viper (Daboia russelli) - This snake is called Tith Polonga in Sinhala and Kannadi Virian in Tamil. Earlier this snake had the scientific name Vipera russelli.

This species is found across most of southern Asia, and the islands of Indonesia, many parts of Pakistan, India and Bangladesh. It is also found further east in Taiwan, Myanmar, Thailand and China.

The Russell's viper has a characteristic pattern on its body. It has a dorsal (upper body) colour of a shade of brown. This colour varies in different snakes.

It also has three rows of oval spots along the body, one along the top of the body and the other two on either side. These spots have a brown center and a dark ring round each spot. It is a very aggressive snake.

The Cobra (Naja naja) - This snake is called Naya in Sinhala and Nalla Pambu in Tamil. The cobra belongs to the genus 'Naja' because of its ability to dilate its broad hood.

Saw-scaled ViperCobras can erect their bodies and spread their hood by erecting the ribs in its neck. Their fangs are short and fixed. The fangs in some snakes are flexible in that the jaws can be moved independently.

Cobras, generally, bite during daytime or as dusk falls and rarely or never by night. As night falls they move into their shelters.

During the day, they lie under grass or jungle undergrowth awaiting their prey, ready to attack at any moment. They also attack when they are disturbed or harmed. Vipers on the other hand are active by night.

This natural behavioural difference between the cobra and the viper has resulted in the Sinhala proverb 'Nayayie polongai vage' (Like cobra and viper) which refers to those who hate each other.

The Common Krait (Bungarus caeruleus) - This snake is called Thel Karawalaya in Sinhala and Katu Viriyan in Tamil.

The two species of krait listed here are distributed in two different climatic and vegetational zones. The Common Krait (Bungarus caeruleus) is the deadliest snake found commonly in the dry zone of Sri Lanka.

The Common krait is mainly confined to the dry zone plains up to about 900 m. It prefers paddy fields, shrub lands and thinly wooded forests.

However, it also lives close to human habitations due the availability of species like frogs and rats on which it also preys. The preferred prey of the krait is the smaller species of snakes.

Kraits are highly venomous and terrestrial snakes. They are nocturnal and move freely about at night.

The Sri Lankan Krait - Endemic (Bungarus ceylonicus) - This snake is called Dhunu Karawalaya in Sinhala and Yettadi Virian in Tamil. This snake is endemic to the island. In both species of the kraits in Sri Lanka, the head and the rest of the body are the same and the beginning of the head is not easily distinguished.

The Ceylon or Sri Lanka Krait inhabits the wetter parts of the intermediate zone and the wet zone up to 3000 feet, and is found in the central hills of Sri Lanka and in the south west quarter of the island

The Saw-scaled Viper (Echis carinatus) - This snake is called Veli Polonga in Sinhala and Suratti Pambu in Tamil.

It is found in Southeastern Arabian Peninsula, southwestern Iran, Afghanistan, Uzbekistan, Turkmenistan, Tadzikhistan, Pakistan, India, Sri Lanka and Bangladesh. Saw-scaled vipers are grey or brownish in colour, patterned on the body with brownish blotches, a wavy white stripe, and a dark cross on their heads.

When aroused, they coil and twist, rubbing their serrated scales against each other to create a sound similar to that of a saw cutting wood and it is this characteristic noise that gives its name.

This is usually a warning given prior to it leaping fast and striking to defend itself. The venom of the Saw-scaled Viper is haemotoxic.

The Slender Coral Snake (Calliophis melanurus) - This snake is called Depath Kaluwa in Sinhala and Pavallap Pambu in Tamil.

This is a highly venomous snake. This species is found in India (Maharashtra), Bangladesh and Sri Lanka. The local race (Calliophis melanuru sinhaleyus) is endemic to in Sri Lanka

Its head, neck and body are of the same width, while its tail and mouth are very small. This snake is about 30 - 50 cm in length and light brown in colour.

Though it has deadly venom, it cannot bite large prey and humans, due to its small mouth. However, there seems to be one case in Sri Lanka, of a death due to a depath kaluwa bite.

Most venomous snakes have cryptic coloration, and thus blend with or simulate some inanimate object of the environment and thereby increase the chances of it being trod on by their prey. This is especially evident in vipers, and is perhaps a good reason for the high incidence of viperine bites.

A typical example is the Hump-nosed viper. The brown, grey and black cryptic coloration of this species camouflages it well amongst dry fallen leaves and other vegetation on the ground.

The resting coiled posture of its body and holding its head elevated at an angle around 45 degrees resembles a dry curled leaf, mainly in the rubber, coffee or cashew (Anacardum occidentale) plantations, where it is common.

The coloration of the Russell's viper and the Saw scaled Viper helps them to merge well with the soil, dry vegetation, rocks and logs where the two species rest during the day.

Similarly, the green, yellow and black colour of the Green pit viper merges into the foliage of the trees, shrubs and creepers on which they rest, resulting in a fair number of people being bitten by green pit vipers while plucking tea leaves, clearing forests, weeding etc.

The following species on which snakes prey are common in human habitations, agricultural fields, and plantations facilitating snake/human contact.

Some of these animals are: frogs (Fejervarya limnocharis, Fejervarya kirtisinghei), both species so common in paddy (rice) fields that they are called paddy field frogs, field mice (Mus booguda), house rats (Rattus rattus), house mice (Mus musculus), field rats (Millardia meltada), house geckos (Gehyra multilata, Hemidactylus frenatus, Hemidactylus brookii etc) skinks (such as Mabuya, Riopa, Lankascincus, Lygosoma), land monitor lizards (Varanus bengalensis), house sparrows and other small birds and poultry. They also take the eggs and young of many bird species

In Sri Lanka, 85% of the circumstances under which snakes have bitten humans was while they were engaged directly or indirectly in agricultural activities or related pursuits such as weeding, preparation of fields, harvesting, guarding crops etc.

In Sri Lanka the majority of farmers live in wattle and daub dwellings with thatched roofs. This is especially so in the dry zone where the Common Krait is widely prevalent.

The walls and the floor of these wattle and daub houses have many cracks and holes which make ideal niches for rats, snakes and other reptiles. Similarly the thatched roofs also offer refuge for animals that are preyed on by snakes.

Seventy four percent of snakebites on humans are inflicted below the knee, while walking on paddy field bunds, tank bunds, foot paths etc. which emphasizes the risk of not wearing footwear and of walking on foot paths in the dark or at dusk without a torch (flashlight).

Farmers, have at various times articulated to me, especially when I was working on the Mahaweli Development Project, that they cannot afford expensive footwear and also that it was not possible to work with ease when clad in boots.

With the clearing of forests, scrub jungles and grasslands, most of the fauna, especially mammals and birds, found themselves restricted to these fragmented and isolated forests.

Many species of snakes have adapted themselves to a life in the cleared lands, agricultural fields, compounds and houses, making the new human settlers also vulnerable to snakebite.

Snakes thrive in this environment, since in these cleared settlements, most of their natural predators, such as wild boar, peafowl, large predatory birds and other natural predators are not present.

On the other hand, due to human activities the animals that snakes prey on, such as rats, mice, amphibians, and other reptiles are found in abundance.

In Sri Lanka, the western medicine and the traditional Ayurveda system are the two popular forms of treatment resorted to by snakebite patients.

The ayurveda medical system has played an important role in the medical care of the people of Sri Lanka prior to the introduction of western medicine.

Most villagers are of the opinion that native treatment is better than western treatment when a snake bite occurs. There is now, however, increased acceptance of western treatment for snakebites.

In western treatment, broad-spectrum antibiotics are used to minimize the risk of local infection, and antivenom injections are given, diluted with isotonic saline, to inactivate the venom.

The advances made by modern medicine have made a tremendous contribution towards effective treatment of snakebite patients.

In the event of a venomous snakebite, medical attention should be sought immediately. While it is helpful to identify the snake, one should not waste time or risk being bitten again by capturing or killing the snake.

The most important things to do if bitten are to stay calm and avoid excessive activity, and seek medical care as soon as possible. Do not make any incisions at the bite. Non-venomous snakebites should be washed with soap and water to reduce the risk of infection.

Snakes are an ecologically important group of animals in our ecosystems performing many essential ecosystem functions. The species that they prey upon can, if not controlled by snakes, lead to the large-scale destruction of our agricultural crops.